Portable electronic equipment including cellular telephones, pagers, laptop computers and a variety of handheld electronic devices has increased the need for efficient voltage regulation to prolong battery life. Bandgap reference bias circuits have long been used to produce reference voltages for voltage regulators and other analog cells. Such circuits typically include a bandgap reference circuit and a startup circuit.
FIG. 1 shows a schematic diagram of an exemplary conventional startup circuit 100 and a bandgap circuit 105. For this example, startup circuit 100 includes transistors 50 and 52 which are configured to produce a logic high voltage at node 54, (their common point of interconnection), whenever the feedback voltage 46 is below the threshold voltage of transistor 50. In other words, whenever the feedback voltage 46 is below the startup voltage threshold, transistor 50 will be off and node 54 will be pulled high by the action of transistor 52. Conversely, when the feedback voltage 46 reaches the threshold voltage of transistor 50, the transistor 50 turns on and pulls down the voltage at node 54. Transistor 52 is a p-channel transistor having its gate coupled to ground, and is therefore always activated. Transistor 50 is an n-channel transistor.
The conventional startup circuit 100 also includes an n-channel transistor 64 which sinks startup current 48 provided by the bandgap circuit 105 when the feedback voltage 46 is below the startup voltage threshold. Conversely, when the feedback voltage 46 is at or above the startup voltage threshold, the transistor 64 is turned off, causing the startup current 48 to cease flowing.
In conventional startup circuits, there is always a current flowing through at least some of the transistors, such as the transistors 52 and 50 in the circuit 100 of FIG. 1, which is detrimental to battery power conservation and bandgap accuracy. When the feedback voltage 46 is above the startup voltage threshold, and if width and length ratios of the transistors 50 and 52 are not well designed, it is possible that the transistor 64 is not fully turned off. Thus, a current leakage occurs which causes the improper operation of the bandgap circuit 105.
In other conventional startup circuits, the startup circuit may be disabled using an external control device. However, such conventional startup circuits do not include an internal circuit that automatically stops the startup circuit when it is no longer needed. Thus, such conventional startup circuits are disadvantageous because they require additional components which may further drain valuable battery power, even when the startup circuit is not needed.
It would be desirable to provide a startup circuit that reduces leakage current from the startup circuit to the bandgap circuit during operation, and to automatically stop current consumption in the startup circuit during periods when it is not needed by the bandgap circuit, without causing unwanted voltage fluctuations.